Electrical thermistor



March 8, 1966 l. B. JOHNS" ETAL 3,239,785

ELECTRICAL THERMISTOR Original Filed Oct. 27, 1961 INVENTORS lRAl. B.JOHNS BERNARD S, WILDI iwa' Baum ATTORNEY United States Patent OfiiicePatented Mar. 8, 1966 3,239,785 ELECTRICAL THERMISTOR Iral B. Johns,Marblehead, Mass, and Bernard S. Wildi, St. Louis, Mo., assiguors toMonsanto Company, St. Louis, Mo., a corporation of Delaware Originalapplication Oct. 27, 1961, Ser. No. 151,131. Divided and thisapplication Jan. 29, 1965, Scr. No.

2 Claims. (Cl. 338-42) This application is a division of applicationSerial No. 151,131, filed October 27, 1961.

This invention relates to organic nitrogenous phosphorus compounds andmore particularly provides certain hydrocarbon-substituted2,4,6-s-triphospha-s-triazines as new and valuable compounds, the methodof preparing the same, and thermistors in which the new compounds areemployed as electric current resistors.

Thermistors are electric current resisting elements made of asemi-conducting material which exhibits a high negative temperaturecoefficient of resistivity. It is well known in the art to employcertain inorganic materials as thermistor elements, e.g., the oxides ofnickel, copper and zinc have been used for this purpose. However few, ifany organic materials have been previously used for this purpose. Formany reasons e.g., for easy fabrication of the thermistor component andfor better control of the manufacturing process to obtainreproducibility insofar as sensitivity and stability is controlled,organic materials are preferable.

Accordingly, an object of the present invention is the provision oforganic compounds having semiconducting properties. Another object isthe provision of an organic compound having a high negative temperaturecoefficient of resistivity to electric current. Still another object ofthe invention is the production of heterocyclic compounds having nuclearphosphorus and nuclear nitrogen. A further object is the provision ofthe heterocyclic triphosphatriazines from readily available, aromatic.phosphorus-halogen compounds and aromatic or alkylaromatic mono-amines.A very important object is the provision of organic thermistors. Afurther object is the provision of new and useful electrical currentregulating devices. Another object of this invention is to provide newand useful thermoregulating devices. These and other objects of theinvention will become apparent as the detailed description of theinvention proceeds.

It has been found that the heterocyclic compounds having nuclearphosphorus and nitrogen and possessing semiconducting properties areobtained by the reaction of certain organic phosphonus dichlorides withcertain organic amines, substantially according to the scheme in which Ris selected from the class consisting of phenyl and alkyl phenyl havingfrom 1 to 5 carbon atoms in the alkyl radical and X is a halogenselected from the class consisting of chlorine and bromine.

When R and R are both the phenyl group, and the reactants arephenylphosphonous dichloride or dibromide and aniline, the product ishexaphenyl-2,4,6-triphosphas-triazine. The lower alkyl-substitutedphenylphosphonous dichlorides or dibromides or the nuclearly alkylsubstituted anilines react similarly. Thus, reaction of 0-, m-, or.p-tolylphosphonous dichloride and aniline give 1,3,5-triphenyl-2,4,6-tri-(o-, mor p-tolyl)-2,4,6-triphosphas-triazine;phenylphosphonous dibromide and o-, m-, or p-toluidine give 1,3,5tri(o-,rnor p-tolyl)-2,4,6-triphospha-s-triazine; o-, mor p-tolyphosphonousdichloride and o-, rnor p-toluidine give hexa-o-, mor p-tolyl-2,4,6-triphospha-s-triazine, p-ethylor p-isopropylphosphonous dichloride andaniline give 2,4,6-tris(p-ethylphenyl)- or 2,4,6 tris(p isopropylphenyl)1,3,5 triphenyl -2,4,6- triphospha-s-triazine, p-butylphenyl-phosphonousdibromide and p-butylaniline give hexakis(p-butylphenyl)-2,4,6-triphospha-s-triazine, p-amylphenylphosphonous dichloride andp-propylaniline give 2,4,6-tris(p-amylphenyl)- 1,3,5 tris(ppropylphenyl) 2,4,6 triphospha 1,3,5- s-triazine,3,4-diethylphenylphosphonous dichloride and 3,4-diethylaniline givehexakis(3,4-diethy1phenyl)-2,4,6- triphospha-s-triazine, andpentamethylphenylphosphonous dichloride and 2,3,4,5,6-pentamethylanilinegive hexakis (pentamethylphenyl)-2,4,6-triphospha-s-triazine.

Reaction of the phenylor alkyl-substituted phosphonous dichloride ordibromide and aniline or nuclearly alkylated aniline takes placereadily, at ordinary or increased temperatures, by simply mixingtogether the two reactants in the presence or absence of an inertsolvent or diluent. Since the reaction takes place with evolution ofhydrogen halide, provision is advantageously made for removal of thehalide as it is formed. This may be done by dephlegmation and/orvigorous agitation, but is generally more expeditiously effected byoperating in the presence of a basic compound as hydrogen chloridescavenger, e.g., the alkali metal or alkaline metal hydroxides such assodium or potassium hydroxide, quaternizing nitrogen compounds such asthe heterocyclic nitrogen bases or the tertiary alkylamines, e.g.,pyridine, quinoline, trimethylamine, triethylamine, etc. Solvents whichare presently useful include the aliphatic or aromatic liquidhydrocarbons such as hexane, benzene, xylene, mineral spirits,chlorinated hydrocarbons such as hexachloroethane, ketones such as2-butanone, ethers or sulfoxides such as dioxane or dimethyl sulfoxideor diethylene glycol dimethyl ether, etc.

Since reaction of the phosphonous dichloride with the amine compoundtakes place very readily, a solvent is generally recommended in order toeffect smooth reaction, and initial mixing of the two reactants isadvantageously conducted either at room temperature or with cooling. Inorder to accelerate completion of the reaction, the reaction mixture maybe heated at, say, the temperatures of from C. to C., and advantageouslyat reflux.

Since condensation of the phosphonous dihalide with the amino compoundto give the present 2,4,6-triphosphas-triazine compounds proceeds byparticipation of 3 moles of the dihalide with 3 moles of the amine, thetwo reactants are advantageously employed in such stoichiometricproportions; however, any excess of either reactant may be used, sinceany unreacted material can be separated from the desired product. Also,if a scavenger for the hydrogen halide be employed, for good yields ofthe desired product the scavenger preferably is employed in a ratiocalculated to be consumed by reaction with the quantity of hydrogenhalide evolved; although, of course, use of physical means of removingthe hydrogen halide from the reaction zone employed in conjunction withthe chemical scavenger will diminish the optimum amount of thescavenger. When using an amine as scavenger, the quaternary ammoniumcompound which is formed is generally insoluble in the organic diluent.Hence, it can usually be separated from the diluent, in which thereactants and product are generally soluble, by filtration ordecantation. The 2,4,6-triphospha-s-triazine product can then berecovered by methods known to those skilled in the art, e.g., bydistilling off the solvent and any unreacted material, by fractionalcrystallization, solvent extraction, etc.

The presently provided hexaphenyl-2,4,6-triphospha-striazine and thelower alkyl derivatives thereof are stable, high-melting compounds whichfind application for a variety of industrial purposes; but owing to theunique property of the present compounds to resist the flow ofelectrical current at low temperatures while permitting current flow athigher temepratures, the triphosphastriazines of this invention areparticularly suitable for the manufacture of thermistors designed foruse in measurement and control apparatus, e.g., temperature and volumeregulating means, oscillators, high frequency power meters, etc.

One of the fundamental differences between most inorganic materials,e.g., metals and semiconductors is the effect of temperature onresistivity. Metals show a slight increase in resistivity withincreasing temperature, while semiconductors show an inverse effect ofmuch greater magnitude. The resistivity of inorganic semiconductorsfollows the relationship p=p exp-AE/kT where p is the resistivity, p aconstant, AE the energy gap between the valence band and conduction bandand AT the product of the Boltzmann constant and absolute temperature.If one plots the log of the resistivity (p) versus the reciprocal of theabsolute temperature (1/ T), a straight line is obtained. The slope ofthis line is (AE/k) and the intercept on the 1/ T axis is log pExperimentally obtained data obtained with inorganic semiconductors fitthe Arrhenius equation. The presently provided 2,4,6-triphospha-1,3,5-triazines are resistors whose temperature variation of resistance islarge enough for useful application. They show the same behavior as dothe inorganic thermistors, with increasing temperature resulting in anexponentially decreasing resistivity.

The invention is further illustrated by, but not limited to, thefollowing examples.

Example 1 Into a 500 ml., 3-necked, round-bottomed flask, equipped withstirrer, reflux condenser, dropping funnel, and nitrogen inlet tube,were placed 9.31 g (0.1 mole) of aniline and 25 g. of triethylamine in300 ml. of freshly distilled benzene which had been dried over sodium.The system was continually swept with dry nitrogen while 17.9 g. (0.1mole) of freshly distilled phenylphosphonous dichloride was addeddropwise, with constant stirring and cooling to 0 C. Immediateprecipitation of by-product triethylamine hydrochloride was evident.After all of the phenyl-phosphonous dichloride had been added, the wholewas refluxed for 1.75 hours. It was then allowed to cool to roomtemperature and filtered to remove the amine hydrochloride. The filteredsolid was washed with benzene, and the combined washings and filtratewere refluxed for 2 hours, after which time the benzene was stripped offat atmospheric pressure. Successive concentrations and filtrations gaveas the third crop of crystals the substantially purehexaphenyl-2,4,6-triphospha-s-triazine, M.P. 262-265" C., and analyzing14.97% phosphorus and 7.16% nitrogen as against 15.57% and 7.03% therespective calculated values for C H PN. Cryoscopic molecular weightdetermination in benzene gave a molecular weight of 590. Testing of theheat stability of the compound in the micro isoteniscope showed it to bestable below 391 C.

Example 2 Employing the equipment described in Example 1 and Workingunder nitrogen, 53.70 g. (0.3 mole) of phenylphosphonous dichloride wasadded dropwise, with cooling, to a mixture consisting of 27.94 g. (0.3mole) of aniline, 65 g. (0.6+ mole) of triethylamine and 600 g. ofbenzene. The whole was then refluxed for a total of 4 hours, allowed tocool, and filtered. The yellow filtrate was concentrated at atmosphericpressure to 180 ml. and allowed to stand overnight. The solids werefiltered off and the filtrate was subjected to three successiveconcentrations and filtrations. There was thus obtained as the fourthcrop of crystals the substantially pure hexaphenyl-2,4,6-triphospha-s-triazine, a high-melting, yellow crystalline solid.

Example 3 Testing of the semiconductor properties of hexaphenyl-2,4,6,-triphospha-s-triazine was conducted as follows:

The temperature resistivity measurements were made in a 4" diametercylindrical quartz cell, the thickness of which could be varied from 0.7to 5 mm. The quartz cell was fitted with a heater, platinum electrodesand a thermocouple. The cell was covered by a bell jar connected to agas inlet vacuum system. The sample of said triphosphatriazine inpowered form was annealed for 2 hours at C. and placed in the cellholder, and the electrodes attached under a constant spring tension. The

ell jar system containing the sample in the cell was evacuated andflushed out with anhydrous nitrogen. The resistivities were determinedwith a megohm bridge (having a range of 10 -10 ohm) at decreasingtemperatures. Eleven determinations were made within a temperature rangeof 50 C. to 111 C. Plotting of the log resistivityreciprocal temperaturedata gave a straight line curve, with a resistivity of 2 10 ohm-cm. atone temperature extreme and a resistivity of 8x10 ohm-cm. at the otherThe log of the resistivity was plotted against 1000/ T and from this theenergy gap AE was arrived at by the Arrhenius equation to be AE=2.49.The intercept log p value was found to be 1.1 10

The data obtained for hexaphenyl-2,4,6,-triphospha-striazine issurprising, for the analogous hexaphenylbenzene showed entirelydifferent resistivity. Thus, using the same testing procedure, at atemperature of 60 C., hexaphenyl-2,4,6-triphospha-s-triazine showed aresistivity of 10 ohm-cm., where hexaphenylbenzene showed a resistivityof 10 ohm-cm. The five orders of magnitude difference between thephosphorus on the one hand and the carbon analogue on the other leads tothe conclusion that the cyclic -PN system shows less resistance to theconduction of an electron or of its opposite, the hole, than does thecyclic C=Csystem. This is probably due to a stereoisomeric positioningof the phenyl radical with respect to the nuclear phosphorus of thetriphosphatriazine system.

The invention provide thermistors comprisinghexaphenyl-2,4,6-triphospha-s-triazine or an alkyl-substitutedderivative thereof in contact with a pair of electric currentconductors. Thermistors comprising the present compounds may be made byforming a compact, rigid tablet or wafer of the compound, volatilizing ametal, e.g., gold, upon portions of the surface thereof to serve ascontact, and positioning wire leads therein. An alternative thermistorconstruction comprises a head of the triphospha-striazine compound heldbetween two wires which serve as leads and, if desired, sealed in glassor other siliceous material. The granular or powderedtriphospha-s-triazine compound may be melted and allowed to fuse arounda pair of electrode tips to provide a very easily manufacturedthermistor. Also, pills of the present compound may be formed, coveredwith a silver-base glaze, and baked or covered with a silicon-basevarnish after the leads are soldered on the coated pills. The presentlyprovided triphosphatriazine compounds may likewise be encapsulate-d in aplastic film, e.g., a film of polyethylene, polyvinyl chloride, nylon ortet-rafluoroethylene polymer and provided with circuit means intimatelysurrounded by the triphosphatriazine and projecting externally. Alaminate thermistor comprising one or more of the present compounds isshown in the sole figure wherein numerals 1 and 2 depict ceramic orglass sheets, numeral 3 depicts ccompactedhexaphenyl-2,4,6-triphospha-s-triazine or a lower alkyl derivativethereof, numerals 4 and 5 are metal contacts which may be of silver orgold, and numerals 6 and 7 are wire leads. Thermistors comprising thepresent compounds are also easily constructed by placing the granulatedor powdered compounds in a hollow rigid casing which may be of ceramicor glass and through which leads are interposed and placed in intimatecontact with the presently provided semi-conducting compounds.

In manufacture of the presently provided thermistors, thetriphosphatriazine compound may be used alone as the only semiconductingcomponent or it may be admixed with known materials which exhibit a highnegative temperature coeificient of resistivity. Also, the presentcompounds may be mixed and pressed with sodium silicate or combined withresinous materials, especially the polymeric silicon-es, to give rigidstructures in wafer, rod, or tablet form.

6 What is claimed is: 1. A thermistor comprising a body portion of acompound of the formula and alkylphenyl having from 1 to 5 carbon atomsin the alkyl radical, means providing an insulated covering on said bodyportion, and electrical leads electrically connected to said bodyportion.

2. A thermistor comprising a body portion ofhexaphenyl-2,4,6-triphospha-s-triazine, means providing an insulatedcovering on said body portion, and electrical leads electricallyconnected to said body portion.

References Cited by the Examiner UNITED STATES PATENTS 3,098,871 7/1963Bezman 260551 OTHER REFERENCES Shaw et al.: Chemical Reviews, vol. 62,pp. 247-252 and 276-277 (June 1962).

RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner.

1. A THERMISTOR COMPRISING A BODY PORTION OF A COMPOUND OF THE FORMULA